Many automotive vehicles with internal combustion engines use mass air flow meters that are located upstream of the internal combustion engine to measure the amount of air flowing into the engine. Responsive to the mass air flow information, the vehicle's engine controller controls fuel flow into the engine to reduce pollution emission levels, increase fuel economy and increase engine performance. To optimize system performance, it is desirable that the mass air flow meter respond quickly enough to supply information representative of the measure of mass air flow into each cylinder of the engine.
One challenge that occurs in attempting to measure mass air flow into the engine is especially prevalent in four cylinder engines. This challenge is air pulses in the intake duct caused by the engine valving during intake and exhaust strokes. Four cylinder engines can generate oscillations of significant amplitude in the intake air flow and engine valve overlap can cause brief periods of flow out of the cylinder intake manifold, causing brief periods of reverse flow in the intake ducts. This bi-directional mass air flow pulsation can cause significant mass air flow meter errors if the mass air flow meter and/or the measurement technique are insensitive to flow direction. Thus, it is advantageous to have a bi-directional mass air flow sensing device especially to control four cylinder engine fueling using mass air flow sensing strategies.
A mass air flow sensor suitable for batch fabrication and associated control circuitry have been described in U.S. Pat. Nos. 4,576,050, 4,713,970, 4,782,708, 5,086,650, and 5,263,380, all assigned to the assignee of this invention.
FIG. 1 illustrates schematically a top view of a typical bi-directional mass air flow sensing device according to these prior patents. The sensing device 14 is centered on a planar substrate such as silicon chip and consists of a central heater 16 and two temperature sensitive resistors (thermistors) 18, 20 located equidistantly upstream and downstream from the heater 16. The two thermistors 18, 20 are at equal temperature in zero-flow conditions but are at different temperatures when fluid flows past sensing device 14. More particularly, the electronic circuit 26 causes heater 12 to generate heat that propagates more toward the downstream thermistor 18, 20 than the upstream thermistor 20, 18 resulting in a positive temperature difference between the downstream and upstream thermistors 18, 20. The sensor output is proportional to the instantaneous temperature difference between thermistors 18 and 20 and increases monotonically with flow magnitude. A negative difference between the downstream and upstream thermistors 18 and 20 occurs during reverse flow conditions. Thus when the sensing device 14 is placed in flow path 12 in duct 10, it is responsive to flow in both the direction indicated by arrow 22 and the reverse direction indicated by arrow 24.
Bi-directional mass air flow sensors based on downstream minus upstream temperature measurements detect reverse flow conditions more accurately than typical sensors based on hot wire measurements. These sensors, however, have a limited frequency bandwidth that is dictated by the physical layout of the sensor and the substrate material selection. The limited bandwidth may introduce errors into the sensor measurement signals that may be significant under high frequency pulsations of air flow through the duct.
In upper frequencies, for example, frequencies over 55-60 Hz, error integrated over a full cycle of air flow pulse may reach over 10% of the actual mass air flow. What is desirable is a sensor whose integrated error is below 10% for a frequency range extending from zero to over 100 Hz because some engines can, under certain operating conditions, create back flow pulses of air through the duct at frequencies in excess of 100 Hz. When a typical prior art bi-directional device according to FIG. 1 measures air pulses having frequencies above 100 Hz, the resulting measurement signals yield errors significantly larger than 10% of the actual mass air flow value, which is unacceptable for precise mass air flow measurement determination.